This invention relates to the detection and quantitative measurement of liquid water droplets entrained in atmospheric air and other gaseous media. It relates, more particularly, to such detection and measurement utilizing a resonant microwave cavity.
Water is present in the atmosphere in three forms: as vapor; chemically bound in solid dust particles many of whose constituents are hydrates; and in the form of condensed droplets. This latter form has been, traditionally, felt to exist only in fogs and clouds when the local humidity approached 100%. Recent research, arising from the intense concern with the properties and activity of contaminants in urban air spaces, has indicated that liquid water is present in substantial quantities even at relatively low water vapor pressures. Such water droplets are generally condensates forming on hygroscopic solid dust particles, and may be present at relative humidities as low as 50%.
These water particles have considerable influence on the physical and chemical acitivty of the atmosphere, and the detection of their presence and the measurement of their mass density in the air are of substantial importance. For example, the small droplets of water refract sunlight and materially contribute to conditions of haze, while their ability to dissolve gaseous chemical species present in the air greatly enhances the chemical activity of the latter.
Methods of the prior art are not readily adapted to the measurement of such particulate liquid water. Measurements made by such methods either lump all water species present -- chemically bound or free, liquid or vapor -- into a single measure of water concentration, or they are sensitive only to the vapor phase, resulting in conventional measurements of relative humidity.
It is, therefore, the primary object of the invention to provide method and means for the detection and measurement of liquid water particles in atmospheric air which is essentially insensitive to the presence of unbound water vapor and to the presence of chemically bound water in solid hydrates.
It is a further object of the invention to provide a detector for liquid water particles in air and other gases which is based on readily reproducible physical phenomena and which relies on the dielectric properties of such water.
It is another object of the invention to provide such a detector wherein the quantitative measurements are based on the changes in the resonant frequency of a microwave cavity.